This is being studied to some degree in Atlanta, GA within Emory University's School of Public Health. They started by looking at playgrounds near intersections and at some point will extrapolate out. Kids will always take a worse 'hit' due to size v. exposure so it will be difficult to make conclusions at this point. I believe Jeremy Sarnat is the Primary Investigator.

I'll say the traffic itself is a bigger threat. Urban Sprawl and Public Health (a book written in part by the CDC Director of Environmental health) states that you're 22 times more like to die in a traumatic accident biking a mile in a city vs. driving a mile in a city. (Obviously, there are some counterpoints to make here.)